Ageing, especially in the West, is a synonym of decline. We're constantly told that what we have become is a negative, that thinning hair, an expanding midriff and blotchy cheeks are evolution's way of declaring: "Don't breed, please!"

But one man's decline is another's desirability. That, anyway, is one of the hopeful messages contained in Dr Richard G Bribiescas' book, How Men Age: What Evolution Reveals About Male Health and Mortality, recently published by Princeton University Press. Bribiescas - professor of anthropology at Yale - doesn't deny the changes that happen to all of us, but puts a new cast on them.

"I propose that the changes that come with age, such as increased fat deposition, facilitate the conversion of testosterone to estradiol [the primary female sex hormone], perhaps making men more amenable to exhibiting parental care. This may have been a way in which older men have been able to continue reproduction at older ages."

Thus, when a slightly chubby man in his sixties drives past in a small, sporty car with a retractable roof, he's trying to compensate for his diminished masculinity. But at the same time, the women seeing him are thinking about his future usefulness as a parent - the very definition of practical masculinity.

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In other words, this chubby man's dad-ness makes him desirable. Hence the rise of the so-called "Dad-bod".

Bribiescas also tackles the sticky problem of men dying earlier than women, which he places in a broader context of our ageing differently throughout our lives. The patterns, he says, are true across any number of cultural contexts and are also very common in other primates and mammals.

The first one becomes obvious when men are young, and are wont to buy blue Subarus, drive fast, get drunk before swimming across rivers and generally behave badly.

"Males tend to exhibit a sharp increase in mortality, roughly between the ages of 15 and 25, primarily due to lowered risk aversion and more risky behaviour. The likely reason for this is to devote more time and effort to procuring mates since these are ages at which females are at peak fertility. High testosterone at younger ages plays an important role."

In a sense, we're still at it 40 or 50 years later.

"There is no male menopause," says Bribiescas. "While male fertility does exhibit declines due to less efficient spermatogenesis, older men - over the age of fifty - still exhibit significant fertility and the ability to father children."

But the crucial aspect of men's ages, and the one that might affect our behaviours all through life, is the biggest one:

"For example, men tend to have higher metabolic rates throughout their lives compared to women due to a greater amount of sexually dimorphic tissue, mostly muscle mass. Higher metabolic rates tend to result in greater cellular, genetic, and tissue damage which contributes to the ageing process and ultimately death.

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The impact of such changes varies enormously depending on where a man lives. In a well-fed society with good medical care, men can weather some of the costs of, say, high testosterone. In a very basic, elemental way, however, men die younger because they have lived fast - and many of those apparent symptoms of decline are merely the price for having fun through the middle years.

We asked Bribiescas to talk us through a few of the classic changes in the male body - and mind - that take place in midlife and beyond.

The responses are, if not quite uplifting enough to straighten backs and tighten bladders, at the very least, distractingly complicated...

The expanding paunch

As more fat is deposited, more testosterone is converted to estradiol which can promote even greater fat deposition. Photo / Getty
As more fat is deposited, more testosterone is converted to estradiol which can promote even greater fat deposition. Photo / Getty

"Testosterone tends to decline with age in well-nourished societies. Testosterone promotes fat burning so when it declines with age, more fat accumulates, particularly around the midsection.

"Fat cells also contain an enzyme called aromatase which converts testosterone to estradiol. This starts a positive feedback loop. As more fat is deposited, more testosterone is converted to estradiol which can promote even greater fat deposition, greater aromatization, and lower testosterone. In addition, estradiol can suppress hypothalamus function, a part of the brain that is involved in testosterone production."

Which all sounds very technical, but is a potentially good way to explain why you're having that extra pint anyway -what's the point of fighting a beer belly, if declining testosterone and rampant estradiol have got it in for your gut anyway? Or, if you're in female company, you can say you're keen to speed up your feminine side - every extra inch gets that estradiol moving nicely.

Grey hair

"This is a process that is not unique to men and multifaceted. One important reason is that as one ages, we become more susceptible to toxic byproducts of aerobic [oxygen-using) metabolism. These toxins are known as reactive oxygen species and the process is called oxidative stress. These toxins damage cells and tissue throughout your body, including hair follicles that contain the cells that give hair their colour."

On the upside, as with the silverback gorilla, the greyness can indicate a kind of serene power and wisdom, plus the ability to manage a harem.

Less hair

"A lifetime of exposure to testosterone and testosterone-like hormones such as dihydrotestosterone tends to damage follicles." In short, your baldness is because you've been horny all your life.

Hunched shoulders

"This primarily due to decreases in testosterone and loss of muscle mass. In addition, declines in testosterone may contribute to osteoporosis in men which can affect posture and skeletal integrity." In short, you're bending over because you're no longer horny.

Weak bladder

"Age-related damage to the prostate contributes to weak bladders, perhaps as the result of a lifetime of high testosterone levels. Other populations, such as Tsimane foragers of Bolivia do not exhibit the same age associated declines in prostate function suggesting that prostate disease and associated symptoms such as weak bladders are not an inevitable part of male ageing and may be susceptible to significant environmental influence.

"The Tsimane tend to have lower testosterone levels because of their under-nutrition and immunological challenges due to tropical disease and chronic infections."

Moving to Bolivia and becoming a forager might seem a drastic solution to all that crossing your legs while driving, stopping in lay-bys and dashing away from anniversary dinners. But it makes for a good bar-room story, to share, no doubt, while ordering another bladder-challenging drink.

Muscle loss

"This is primarily due to declines in testosterone. The cells in the testes that produce testosterone become less effective. Interestingly, if older men from well-fed industrialized societies are provided testosterone supplements, their muscle growth tends to respond in a similar fashion as younger men given the same doses. This suggests that muscle loss centres around changes in hormone levels and not necessarily changes in muscle responsiveness."

So, either comfort yourself with the knowledge that your muscles are potentially still there, but that your dwindling sex-drive means you don't really care - or, it's time to consider those testosterone supplements.

Sagging moobs

"Muscle loss contributes to sagging and increases in estradiol can promote gynecomastia or the growth of breast tissue". It's time to consider those testosterone supplements.

Grumpiness

"I would argue that not all older men are grumpy. Declines in health, well-being, and increases in various aches and pains can make anyone grumpy!"

So, even Dr Bribiescas, after all, uses the "decline" word. He also sounds just a little bit grumpy in this response. Perhaps I've oversimplified his thesis. In any case, evolution over thousands of years might account for some of the changes that take place in men over three score and ten - but that won't necessarily help you when you're unlucky enough to catch yourself naked in the mirror, or at the GPs, or, rather, not at the GPs, because, as a man, you're more likely to entrust Wikipedia with your age-related self-doubts - which in turn, may lead to an even earlier death.